Radar level gauges are in wide use for measuring process variables of a product contained in a tank, such as filling level, temperature, pressure etc. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards the product by a probe acting as a waveguide. The probe is generally arranged vertically from top to bottom of the tank. The electromagnetic signals are subsequently reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
In many applications, it is also desirable to be able to measure the density of a product in a tank in order to further improve the accuracy during custody transfers. Due to the large amount being transferred during custody transfers, a small measurement error will incur large economical costs. Another desire is to survey a possible vertical stratification of the density. In tanks for liquid gas (LNG, LPG etc.) suck stratification might create unstable conditions. A number of density measurements at different heights are needed to provide that information.
WO2006084263 discloses a system for sensing, monitoring and evaluating properties of fluids used in fluid systems. In particular, mechanical resonators are arranged at multiple positions within the fluid system. The mechanical resonators can be actively stimulated by an external signal and the response from the resonators can be used to determine for example the density of the product in which a resonator is located. The mechanical resonators are specified to resonate in the kHz to MHz range, and are configured as tuning forks to vibrate at a resonance frequency to determine properties in the fluidic system.
However, mechanical resonators are very sensitive to contamination, i.e. if a contaminant is attached to a tuning fork-type resonator, the resonance frequency of the resonator may change due to the additional mass of the resonator, which in turn may lead to an erroneous determinations of a property of the fluid. Accordingly, it is desirable to provide an improved system for determining properties of a product kept in a tank.